11 Bio Chapter Summary

Photosynthesis

Chapter Summary

  • Photosynthesis is a process in which the plants use light energy to make carbohydrate from carbon dioxide and water.
  • Photosynthesis occurs in two stages, viz. light reaction and dark reaction. The light reaction is light-dependent.
  • Dark reaction is light-independent reaction. Dark reaction is utilised to capture and reduce carbon dioxide. Dark reaction doesn’t mean that it happens in the absence of light.
  • Chloroplast is the cell organelle where photosynthesis takes place in plants and algae. A typical plant cell may contain about 10 to 100 chloroplasts.
  • Chlorophyll is the main pigment to absorb light. Additionally, carotenes and xanthophylls are also used by plants to absorb light energy. Algae also use chlorophyll for absorbing light.



  • The light reaction is also called the Photochemical phase. It includes light absorption, water splitting, oxygen release and the formation of high-energy chemical intermediates (ATP and NADPH). Many complexes are involved in the process.
  • Water is split into H+, [O] and electrons. The splitting of water is associated with PS II. This creates oxygen. Photosystem II provides replacement for electrons removed from PS I.
  • Synthesis of ATP from ADP and inorganic phosphate in the presence of light is called photophosphorylaton.
  • When the two photosystems work in a series; first PS II and then the PS I; a process called non-cyclic photophosphorylation occurs.
  • When only PS I is functional, the electron is circulated within the photosystem and the cyclic flow of electrons leads to phosphorylation.
  • The products of light reaction are ATP, NADPH and O2.
  • The Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.
  • Fixation of CO2 into a stable organic intermediate is called carboxylation. In this step, carbon dioxide is utilised for the carboxylation of RuBP.
  • Reduction involves utilization of 2 molecules of ATP for phosphorylation and two of NADPH for reduction of each CO2 molecule fixed. For the removal of one molecule of glucose from the pathway, fixation of six molecules of CO2 and 6 turns of the cycle are required.
  • Regeneration involves regeneration of CO2 acceptor molecule RuBP. This is necessary for the cycle to continue without interruption. This step requires one ATP for phosphorylation to form RuBP.
  • Thus, for each CO2 molecule entering the Calvin Cycle, 3 molecules of ATP and 2 of NADPH are required.
  • Plants which are adapted to dry tropical regions use the C4 pathway. While 3-PGA is the first carbon fixation product in C3 plants, it is oxaloacetic acid (4 carbon atoms) which is the first carbon fixation product in C4 plants.
  • In C4 plants, large cells are found around the vascular bundles. These are called sheath cells. Leaves have special anatomy; called Kranz anatomy.
  • RuBisCO is the most abundant enzyme in the world. Its active site can bind to both CO2 and O2. But RuBisCO has a much greater affinity for CO2 than O2. The relative concentration of O2 and CO2 determines which of them will bind to the enzyme.
  • Factors which affect photosynthesis are; light, carbon dioxide, temperature and water.